To aid and abet is to intentionally and knowingly assist in or encourage the commission of a crime. Generally speaking and aider or abettor is considered a party to the offence, compared to the perpetrator who is the principle. Whilst jurisdictions vary, many common law jurisdictions apply a maximum sentence that is exactly half that of a principle offender.

An aider or abettor can be contrasted with an accomplice to a crime (such as a getaway driver) who will generally be considered a principle offender. The distinction arises from the passive nature of the encouragement or assistance given.